Method and apparatus for maintaining product availability during a disturbance in an air separation unit

ABSTRACT

A method for operating an air separation unit during an unexpected disturbance is provided. The method can include the steps of: determining that a process disturbance has occurred; starting-up a liquid back-up system that is configured to deliver a product gas at a desired product pressure; and introducing compressed air from an air accumulator into the air separation unit at a location that is downstream a main air compressor and upstream a cold box, wherein the compressed air is introduced in an amount that is effective for maintaining nominal operation of the air separation unit during the process disturbance and until the liquid back-up system is delivering the product gas at the desired product pressure.

TECHNICAL FIELD

The present invention generally relates to a method for providing an air gas stream in the event of a disturbance in an air separation unit. The method is particularly useful for maintaining product purity of air gas streams (e.g., nitrogen, oxygen, argon, etc. . . . ) during the transient phase from normal production until a liquid backup system can be brought online in the event of a trip of the plant or reduced flows of incoming air feed.

BACKGROUND OF THE INVENTION

A typical air separation unit (ASU) may include use of 1) Main Air Compression (MAC) to medium pressure ˜6 bar, 2) purification unit, 3) Booster Air Compression (BAC) to high pressure ˜50-60 bar, 4) main heat exchanger, and 5) medium pressure, low pressure and argon distillation columns for the production of oxygen, nitrogen, and argon. Various arrangements for the MAC include a single large machine (1×100%), two half-sized machines (2×50%), or combination of the MAC and BAC into a single machine. One economical arrangement is 1×80% MAC, which can be combined with the BAC plus a second 1×20% MAC. This 1×20% MAC can be either new equipment, existing equipment, or from a nearby pipeline.

Also in this ASU for producing high-pressure gaseous oxygen (GOX), a typical choice of process configuration can include an internal compression cycle, in which liquid oxygen (LOX) is produced in the sump of a low-pressure column and is pumped to high pressure and vaporized in the main heat exchanger. Vaporization heat duty is partially provided by condensing pressurized air in the same main heat exchanger.

In order to ensure the product availability to the end user of the gaseous product, particularly in the event of ASU trip, the facility is usually equipped with a LOX backup system as a secondary product supply. In the LOX backup system, LOX is pumped from a backup liquid storage and is vaporized in a vaporizer exchanger to continue the supply of gaseous product. GOX backup systems are also available; however, they are less used than the LOX backup system for economic reasons.

When producing large amounts of oxygen, argon, and nitrogen, distillation of air at cryogenic temperature is often the most economical choice. Process upsets can greatly affect the operation of the distillation column, since the purity profile inside the column changes, which ultimately affects the final product purity. One example of such a process upset is a sudden change in airflow to the cold box (e.g., trip of the 1×20% MAC), which disrupts the purity profiles in the various distillation columns followed by the loss of product purities.

A complete ASU trip or one of multiple MAC trips inevitably leads to either a complete outage of product or rapid reduction of production capacity. As used herein, a complete trip means all MAC's trip simultaneously (e.g., air flow from the compressors to the cold box becomes zero) and product flows from the air separation unit quickly goes to zero, possibly immediately. One MAC trip means one MAC has a flow of zero while at least one MAC is still in operation in a multi-MAC arrangement, so that air to the cold box and product from cold box are reduced.

As the product stream is likely being fed either directly to a downstream user or to a pipeline, a change in product purity and/or flow rate can have dire consequences for the downstream user. Therefore, in order to minimize the upset or avoid a trip of the downstream user's process during production downtime, the downstream user often imposes a stringent requirement on allowable flow and/or pressure deviation during the very short transition period (often in order of seconds) from normal ASU operation to backup system. As noted above, liquid backup systems have an inherent delay, and therefore, cannot be the only design used in instances where the product purity and pressure must be maintained at all times. In those instances, an additional costly online gas buffer is often required.

Therefore, it would be beneficial to provide a process and apparatus that could provide the responsiveness of a gaseous backup system while also being more economically feasible.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a device and a method that satisfies at least one of these needs. The objective of the current invention is to be able to adjust airflow to the cold box in a gradual and controllable manner during a process upset related to air flow supply in order to maintain the distillation column purity profiles. By maintaining operation of the cold box during the time it takes to transition to an alternative operating model (as a non limiting example, the liquid backup system to become operational), the availability of product is ensured during the transitional phase from normal production to either (a) reduced flow rate production without additional backup system and/or (b) normal production rates utilizing an additional backup production.

In one embodiment, this can be achieved by including a front-end air accumulator that will bleed the air into the cold box in such manner as to minimize the rate of change in airflow to the process in the event of a process disturbance. In certain embodiments, when a compressor trips resulting in a sudden loss of airflow, the valve from the front-end air accumulator opens followed by its gradual closing while the distillation columns are ramped down, thus producing reduced stable flow rate loads to the customer/pipeline without loss (or with a minimal loss) of purity and/or pressure.

In another embodiment, when a compressor trips resulting in a sudden loss of air flow, the valve from the accumulator opens while the backup system (typically liquid vaporized from storage) is started and ramped up. Alternatively, without an additional backup system and without multiple air compressors, the accumulator allows temporary production while downstream customer systems are shutdown.

In one embodiment, a method for operating an air separation unit during an unexpected disturbance is provided. The method can include the steps of: determining that a process disturbance has occurred; starting-up a liquid back-up system that is configured to deliver a product gas at a desired product pressure; and introducing compressed air from an air accumulator into the air separation unit at a location that is downstream a main air compressor and upstream a cold box, wherein the compressed air is introduced in an amount that is effective for maintaining nominal operation of the air separation unit during the process disturbance and until the liquid back-up system is delivering the product gas at the desired product pressure.

In optional embodiments of the method for operating the ASU:

-   -   the air accumulator is filled with compressed air that was         previously compressed in the main air compressor;     -   the air separation unit further comprises a second main air         compressor;     -   the second main air compressor is intentionally turned down         during periods of high electricity costs and air from the air         accumulator is introduced into the air separation unit in an         amount that is effective for maintaining nominal operation of         the air separation unit while the second main air compressor is         taken off-line;     -   the air separation unit further comprises a booster air         compressor;     -   the process disturbance is selected from the group consisting of         a trip of the main air compressor, the booster air compressor,         and combinations thereof;     -   the process disturbance is determined by a drop in air pressure         or process flow rates;     -   the air accumulator has an air pressure that is above a         discharge pressure of the main air compressor; and     -   the air accumulator has a capacity that is sufficiently large to         allow for operation of the air separation unit for the period of         time it takes the liquid back-up system to deliver the product         gas at the desired product pressure.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a process flow diagram of an embodiment of the present invention.

FIG. 2. is a process flow diagram of another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention allow the ability to continue to send compressed and purified air to the cold box even after a potential MAC trip for a period of time. This can be useful for allowing (a) gradual and more controlled transition to reduced production load without a backup system, (b) normal air flow rates to cold box during gradual startup of a backup system, or (c) gradual shutdown of production to customer, thereby allowing the customer a more controlled shutdown process.

As noted previously, ASUs can have various MAC arrangements—including single or multiple MACs, with different load configurations. As expected, in instances in which a MAC that carries a lower overall load (e.g., 20% of total air flow), the perturbation of the overall process is less as compared to an instance where the 80% MAC tripped or even if the MACs were split evenly (e.g., 50%) and one tripped. As such, those of ordinary skill in the art will recognize that the volume of the air accumulator will need to be adjusted accordingly.

In a preferred embodiment, a MAC trip signal will trigger a ramp down of airflow in an acceptable rate in order to maintain the product purities. During this transient time of switching from cold box operation to the liquid backup system, high-pressure air from the air accumulator is introduced into a selected location of the cold box by opening one or more valves. In a preferred embodiment, the air accumulator can either be refilled by the air from the discharge of the MAC (˜4 to 7 bar), the discharge of the BAC (˜50 bar) or the discharge of the booster of the turbine (˜65 bar) or any other external source.

In the event of a process disturbance (e.g., an ASU trip, a MAC trip, and/or a BAC trip) and during transient phase until the full backup gas supply, the ASU LOX pump can continue to supply LOX thanks to the liquid inventory in the column to the ASU main exchanger where the LOX is vaporized. The vaporization heat duty is provided by condensing air being bled from the air accumulator into the passage in the main exchanger where the pressurized air is condensed in normal operation. This allows for the production level from the ASU to be maintained temporarily during the transient phase until the liquid backup system reaches its full capacity and at proper delivery temperatures/pressures.

In certain embodiments, the purified and compressed air from the air accumulator serves as a temporary process air supply, either to downstream of the front-end air purification unit, or to downstream of the discharge of turbine booster, or both mentioned locations simultaneously in the event of MAC, BAC or a complete ASU trip.

In one embodiment, the air pressure in the air accumulator can be higher or the same or below the pressurized air operating pressure. However, the air pressure is preferably higher than the MAC discharge pressure. In one embodiment, the air accumulator can be refilled by either internal pressurized air or an external air source.

The capacity of the air accumulator is preferably determined by the amount of air required to ensure a smooth transition of ASU operation from high air flow to turn down and to maintain product supply during the transition from ASU supply to liquid backup system supply.

Referring to FIG. 1, first air stream 2 is compressed in first MAC 10 and then purified in front-end purification (FEP) unit 30 to form purified air stream 32. In the embodiment shown, purified air stream 32 is sent to cold box 35, which includes a main heat exchanger and distillation column system. While within cold box 35, purified air stream 32 is cooled to a temperature suitable for cryogenic rectification of air and then introduced into a distillation column system for separation therein. An oxygen-enriched stream and a nitrogen-enriched stream are withdrawn from the cold box 35.

In the embodiment shown, a portion of the compressed and purified air stream 32 can be sent to air accumulator 90. In another embodiment not shown, air accumulator 90 can be filled with compressed and purified air from an external source.

In the event of a sensed process disturbance (e.g., MAC trips), the liquid back-up system (not shown) will be started up. However, as described above, the liquid back-up system is not capable of instantaneously producing gaseous product at proper flow rates and pressure. In the embodiment shown in FIG. 1, since the MAC is no longer delivering an appropriate flow of air to the system, valve 95 will be opened and a flow of purified and compressed air from air accumulator 90 will be introduced to a location that is downstream MAC 10. As this valve can be opened very quickly, certain embodiments of the invention allow for the continued operation of the ASU during an unplanned process disturbance for the transient period to get the liquid back-up system operational.

FIG. 2 provides another schematic representation of an embodiment of the present invention, in which the ASU also includes a booster air compressor 60 and a turbo booster 70, 80. Referring to FIG. 2, first air stream 2 is compressed in first MAC 10 and second air stream 4 is compressed in second MAC 20, before being combined together to form compressed air stream 12. Compressed air stream 12 is then fed to front-end purification unit (FEP) 30 to remove components that might freeze at cryogenic temperatures (e.g., water and carbon dioxide).

In the embodiment shown that includes booster air compressor 60, purified air stream 32 is split into a first portion 34 and a second portion 36. First portion 34 is kept at substantially the same pressure as the discharge of the MAC (minus pressure losses inherent in piping and equipment) and then introduced into a warm end of the main heat exchanger 40. After cooling in main heat exchanger 40, cooled first stream 42 is then introduced into distillation column system 50 for separation therein.

In embodiments having BAC 60, second portion 36 is further compressed with a first boosted fraction 62 being cooled in main heat exchanger 40, and then withdrawn at an intermediate location and then expanded in turbine 80 to form expanded air 82, which is then introduced to distillation column system 50 for separation therein. Second boosted fraction 64 is further compressed in warm booster 70 to form boosted stream 72. The embodiment shown preferably includes cooler 71 in order to remove heat of compression from boosted stream 72 prior to introduction to main heat exchanger 40. In the embodiment shown, warm booster 70 is coupled to turbine 80, thereby forming what is commonly referred to as a turbo-booster, which allows for the spinning of the turbine 80 to spin the warm booster 70.

Boosted stream 72 can then be sent to main heat exchanger 40 for cooling, wherein the resulting cooled boosted stream 44 is withdrawn from heat exchanger 40 and expanded across a Joule-Thompson valve 45 to produce additional refrigeration for the system before being introduced to the distillation column system for separation therein.

In the embodiment shown, distillation column system 50 is configured to provide a waste nitrogen stream 51, a medium pressure nitrogen stream 53, a low-pressure nitrogen stream 55 and a high-pressure gaseous oxygen stream 57. In the embodiment shown, liquid oxygen 52 is withdrawn from the sump of the lower-pressure column (not shown) and pressurized in pump 100 before being heated in main heat exchanger 40 to form gaseous oxygen stream 57. Argon product 59 can also be withdrawn from the distillation column system.

Air accumulator 90 can be fluidly connected to the system at any point located downstream of MACs 10, 20. In the embodiment shown, there are three different locations that air accumulator 90 is connected, with valve 91 being connected just downstream FEP unit 30, valve 93 being connected just downstream booster compressor 60, and valve 95 being connected downstream the warm booster 70. A fourth location could be upstream the FEP unit 30 (not shown).

As with FIG. 1, in the event of a detected disturbance (e.g., pressures or flow rates of feed air streams are outside of set points), one of the valves (91, 93, 95) are opened in order to allow for air from air accumulator 90 to enter the ASU system. As with FIG. 1, this allows the ASU to continue to operate even in the event of a partial or complete loss of compressed air stream 12—thereby giving the associated liquid back-up system (not shown) time to become operational.

Embodiments of the present invention therefore allow for continued operation of the ASU in the event of a process disturbance for at least a period of time that is sufficient to bring the liquid back-up system online.

In the includes a double column system (not shown) in combination with an argon column (not shown)

Those of ordinary skill in the art will recognize that the distillation column system 50 can be any column system that is configured to separate air into at least a nitrogen-enriched stream and an oxygen-enriched stream. This can include a single nitrogen column or a double column having a higher and lower pressure column, as is known in the art. In another embodiment, the distillation column system can also include other columns such as argon, xenon, and krypton columns. As all of these columns and systems are well known in the art, Applicant is not including detailed figures pertaining to their exact setup, as there are not relevant to the inventive aspect of the present invention.

Moreover, while the embodiments shown in the figures shows one or two MACs, the invention is not intended to be so limited. Rather, certain embodiments of the invention are intended to cover any configuration of MAC(s), whether there be one, two or more MACs.

Moreover, those of ordinary skill in the art will recognize that various means for identifying process disturbances are well known in the art. As non-limiting examples, process disturbances can be identified by employing various pressure indicators, flow indicators and the like. For example, if the pressure or flow of the air coming out of one of the MACs is below an operational set point, a process controller could identify that at least one of the MACs is not operating properly, and then start the backup sequence as described above (i.e., start the LOX backup system while introducing air from the air accumulator into the appropriate point within the system).

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step or reversed in order.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

What is claimed is:
 1. A method for operating an air separation unit during an unexpected disturbance, the air separation unit having a main air compressor and a cold box, the method comprising the steps of: determining that a process disturbance has occurred; starting-up a liquid back-up system that is configured to deliver a product gas at a desired product pressure; and introducing compressed air from an air accumulator into the air separation unit at a location that is downstream the main air compressor and upstream the cold box, wherein the compressed air is introduced in an amount that is effective for maintaining nominal operation of the air separation unit during the process disturbance and until the liquid back-up system is delivering the product gas at the desired product pressure.
 2. The method of claim 1, wherein the air accumulator is filled with compressed air that was previously compressed in the main air compressor.
 3. The method of claim 1, wherein the air separation unit further comprises a second main air compressor.
 4. The method of claim 1, wherein the second main air compressor is intentionally turned down during periods of high electricity costs and air from the air accumulator is introduced into the air separation unit in an amount that is effective for maintaining nominal operation of the air separation unit while the second main air compressor is taken off-line.
 5. The method of claim 1, wherein the air separation unit further comprises a booster air compressor.
 6. The method of claim 1, wherein the process disturbance is selected from the group consisting of a trip of the main air compressor, the booster air compressor, and combinations thereof.
 7. The method of claim 1, wherein the process disturbance is determined by a drop in air pressure or process flow rates.
 8. The method of claim 1, wherein the air accumulator has an air pressure that is above a discharge pressure of the main air compressor.
 9. The method of claim 1, wherein the air accumulator has a capacity that is sufficiently large to allow for operation of the air separation unit for the period of time it takes the liquid back-up system to deliver the product gas at the desired product pressure. 